pmic: pca9450: add DM_I2C dependencies in Kconfig

The pca9450 driver uses dm_i2c_{read,write}, which
are (unsurprisingly) only available with DM_I2C. Make sure one can't
create an unbuildable .config by adding proper dependencies.

While here, append "in SPL" to the prompt for the SPL_ variant so it
doesn't read the same as the one for the non-SPL_ variant.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>